The present invention concerns image or scene projection arrays, particularly infrared scene projectors and related control electronics.
Analogous to digital slide projectors, scene projectors are devices that generate images based on input electrical signals. Infrared scene projectors, a particular class of scene projectors, generate images in the infrared portion of the electromagnetic energy spectrum. These type scene projectors are useful in testing infrared surveillance and guidance equipment which identify and track objects, such as enemy missiles, based on their infrared emissions or heat-generation characteristics.
Infrared scene projectors typically include a thermal pixel array which converts electrical input signals into a desired thermal or infrared image. The thermal pixel array is typically a rectangular array of hundreds of interconnected infrared pixels, with each pixel having a unique row and column position in the array. Each pixel, which includes a pixel control transistor and a resistor coupled between positive and ground voltage terminals, converts an electrical input voltage into a corresponding amount of heat, or thermal energy, based on the magnitude of the input voltage. Thus, by precisely controlling the input voltage, one controls the infrared emission of each pixel and thus defines an infrared image with desired characteristics.
One problem with conventional infrared scene projectors concerns how precisely one can specify the infrared emission at each pixel in typical thermal pixel arrays. In particular, thermal pixels arrays operate with large electric currents. As these large currents flow through the ground terminal (ground bus) which is distributed throughout the array, large voltages (based on the electrical resistance of the ground terminal and the current magnitude) develop at various points along the ground terminal, meaning that the ideally constant ground voltage actually varies from point to point on the ground terminal and therefore from pixel to pixel. Thus, each pixel has a unique ground voltage based on its point of connection to the ground terminal. This means, for example, that applying the same pixel control voltage to two pixels in the array would not produce two identical infrared emissions as desired, but would instead produce two significantly different infrared emissions based on the unique ground voltages of the pixels.
Conventionally, there have been two approaches for addressing this problem. One approach entails estimating the ground voltage at each pixel and adjusting the pixel control voltage for each pixel to account for its specific ground voltage. The other approach entails biasing each pixel control transistor to operate in its saturation region and minimizing resistance of the ground bus, so that the pixels are less sensitive to ground voltage. However, neither approach has been completely successful in overcoming the ground-variance problem, since with identical pixel control voltages, pixel brightness still varies significantly across infrared pixel arrays.
Accordingly, there remains a need for better solutions to the ground-variance problem in infrared pixel arrays.
To address this and other problems, the present inventors devised not only innovative infrared pixel arrays and related control methods, but also infrared scene projector incorporating them. One exemplary infrared pixel array includes two or more pixels, with each pixel including a resistor and its own current-mirror circuit. A related control method entails receiving a pixel-control voltage, converting the pixel-control voltage to a current, and then coupling the current to the current-mirror circuit in one of the pixels. The current mirror applies an appropriate control current to the resistor and thus enables operation of the pixel independent of ground-voltage variations.